1. Field of the Invention
The present invention relates to an optical scanning apparatus and an image forming apparatus and, more particularly, to a color image forming apparatus such as a laser beam printer, a digital copying machine, or a multifunction printer, which has an electrophotographic process.
2. Description of the Related Art
Conventionally, an optical scanning apparatus is used in a laser beam printer (LBP), a digital copying machine, a multifunction printer, or the like. In this optical scanning apparatus, a light beam optically modulated and emitted by a light source is periodically deflected in accordance with an image signal by, for example, a deflector formed from a rotating polygon mirror. An imaging optical system having the f−θ characteristic condenses the deflected light beam to a spot on the photosensitive surface of a photosensitive member (photosensitive drum) and optically scans the photosensitive surface, thereby recording an image.
As an optical scanning apparatus used in a color image forming apparatus, there is known an optical scanning apparatus employing a system (sub-scanning oblique incident system) that causes a plurality of photosensitive drums to share one deflector and makes an light beam obliquely enter the deflecting surface of the deflector in the sub-scanning section (FIG. 12A). According to the sub-scanning oblique incident system, a plurality of light beams toward the photosensitive surfaces can be separated without making the deflecting surface of the deflector large in the sub-scanning direction.
Japanese Patent Application Laid-Open No. 2007-178748 discloses an optical scanning apparatus employing a sub-scanning oblique incident system in which the spot position on the lens surfaces (incident surface and exit surface) of a lens included in the imaging optical system forms a scanning locus across the lens center line (a line that passes through the center reference position of the lens surfaces and extends in the main scanning direction). Japanese Patent Application Laid-Open No. 2004-361941 discloses an optical scanning apparatus that tilts the plane normal of each lens surface and changes the tilt in the main scanning direction in the sub-scanning section.
When plastic-molding an imaging optical element included in the imaging optical system used in the optical scanning apparatus, the following manufacturing method is used. That is, the method includes the steps of making a mold based on a design value, evaluating the optical performance of the lens surface of an imaging optical element molded using the mold, correcting the mold based on the result of optical performance evaluation, and molding an imaging optical element using the corrected mold. In the optical performance evaluation, the shape of the lens at the light beam passing position is evaluated.
An optical scanning apparatus used in a monochrome image forming apparatus does not employ the sub-scanning oblique incident system. For this reason, the light beam scans on a line (meridian line) that passes the origin of an aspherical surface formula defining the lens surface shape in the sub-scanning section including the optical axis of the imaging optical system and is extended in the main scanning direction. That is, it is possible to easily evaluate the optical performance (evaluate the lens shape on the meridian line).
In a color image forming apparatus, however, beams pass positions far apart from an origin position CL of an imaging optical element 7A, as shown in FIGS. 12B and 12C. In addition, as shown in FIG. 13, the beams pass through scanning loci largely curved on the lens surface of the imaging optical element 7A. Note that in FIG. 12C, each lens surface is tilted and decentered, and the origin position CL is set in a plane P0 that passes the center of a deflecting surface 5a in the sub-scanning direction and is perpendicular to the rotation axis of a deflector 5. In FIG. 12B, the origin position CL is shifted and decentered from the plane P0 in the sub-scanning direction by a decentering amount ΔZ.
When the beams pass the positions far apart in the sub-scanning direction from the origin position CL of the lens surface, as described above, it is not possible to easily evaluate the optical performance. It is necessary to correctly measure, for example, the meridian line shape of the lens, the radius of curvature (the radius of curvature of the sagittal line) in the sub-scanning section, the tilt amount of the lens surface, the non-arc amount of the sagittal line, and the like and evaluate the optical performance in consideration of all of them. The lens surface shape evaluation at the beam passing position is possible only when the plurality of items are correctly measured. Hence, the mold correction accuracy lowers, and the number of times of correction increases.